Conventionally, thermoelectric generation of electricity is known in which the thermoelectric elements are disposed between a heat exchanger at a higher temperature part and another heat exchanger at a lower temperature part to generate electricity. The thermoelectric element is an application of a thermoelectric effect to be called Seebeck effect. In the case where a semiconductor material is used as a thermoelectric material, the thermoelectric power module is configured by electrically connecting a thermoelectric element formed of a P-type semiconductor thermoelectric material and another thermoelectric element formed of an N-type semiconductor thermoelectric material via an electrode.
Such a thermoelectric power module has a simple structure and can be easily treated, and stable characteristics can be retained. Therefore, research work thereof has been widely progressed toward application for the thermoelectric generation of electricity in which electricity is generated by utilizing heat in a gas discharged from an engine of a car, a furnace of a factory, and so on.
Generally, the thermoelectric power module is used in a temperature environment where a difference between a temperature (Th) at a higher temperature part and a temperature (Tc) at a lower temperature part becomes large in order to obtain high thermoelectric conversion efficiency. For example, a thermoelectric power module employing a typical bismuth-tellurium (Bi—Te) based thermoelectric material is used in a temperature environment where a temperature (Th) at the higher temperature part becomes 250° C. to 280° C. at maximum. Accordingly, deterioration of a joint layer for joining the thermoelectric element to the electrode becomes a problem.
As a related art, Japanese patent application publication JP-P2012-231025A, especially, paragraphs 0017-0018 discloses a thermoelectric module in which oxidation is prevented from generating at a joint boundary of the thermoelectric module and loss of output power is small. The thermoelectric module consists of thermoelectric elements formed of a metal oxide and for converting difference in temperature into electric power, electrode members for extracting the electric power converted by the thermoelectric elements, and conductive joint layers for joining the thermoelectric elements to the electrode members, and is characterized in that the joint layers have oxygen infiltration preventing means for preventing oxygen from infiltrating into the joint layer.
It is effective to prevent oxygen from infiltrating into the joint layer in order to prevent deterioration of the joint layer. However, according to FIG. 1 of JP-P2012-231025A, only infiltration of oxygen into a joint boundary between the conductive joint layer and the thermoelectric element is prevented by a film, and therefore, deterioration of the conductive joint layer itself cannot be avoided. Further, there are other factors, which deteriorate the thermoelectric power device, such as a short circuit due to extraneous materials, dew condensation, and so on, and therefore, it is insufficient to provide the oxygen infiltration preventing means for preventing oxygen from infiltrating into the joint layer. Furthermore, the temperature range in which the thermoelectric element formed of a metal oxide displays its ability is a temperature range equal to or higher than 500° C. which is different from a usual temperature range for the thermoelectric generation of electricity. Accordingly, an application range of the thermoelectric power device is narrowed.
Further, Japanese patent application publication JP-P2004-228293A, especially, paragraphs 0007-0009 discloses a thermoelectric module provided with an oxidation preventing film which enables use of a thermoelectric element even in high temperature applications such as a car. The thermoelectric module is characterized in that a thermoelectric material of the thermoelectric module is coated with an inorganic and organic highbred material.
However, a main factor, which deteriorates performance of the thermoelectric power module, is deterioration of a joint layer for joining a thermoelectric element to an electrode, and therefore, it is insufficient to only coat the thermoelectric material. Further, since no measure is taken to prevent the oxidation preventing film from peeling off, there is a possibility that the oxidation preventing film may peel off the thermoelectric material to lose the oxidation preventing function. On the other hand, if the oxidation preventing film is made thick, heat-leakage between the heat exchanger at the higher temperature part and the heat exchanger at the lower temperature part will increase.